Watch Out for that Last Step Watch Out for that Last Step


Texas A&M University


Papanikolas, El-Guindy

The students learn cryptography by studying successively more complicated encryption algorithms. The first method is In subsequent methods, each letter is converted to a number and converted back at the end. And to simplify computations, many methods are initially done with a 5 letter alphabet, A=1, D=2, I=3, M=4 and blank=0, to spell things such as "Madam Im Adam". The second method is The third method is Next the students are asked what changes if one uses a 6 letter alphabet. By looking at the multiplication table, they discover that not every number has an inverse and this is because 6 is not prime. So they conclude you need to work with a prime modulus.

Finally the students are asked how to make it more difficult to decrypt the message. Hopefully, they conclude that you should use a larger prime modulus and encrypt several letters at a time.

© Philip B. Yasskin, 2005.
Last updated Jan 22, 2005